[media] vb2: Merge vb2_internal_dqbuf and vb2_dqbuf

After all the code refactoring, vb2_internal_dqbuf is only called by
vb2_dqbuf.

Since the function it is very simple, there is no need to have two
functions.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>